1. Field
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning means can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or which covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through the beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
At present, lithographic processes are used in particular to form devices, such as integrated circuit devices, that have very small patterned features. There is a continuing demand to reduce the size of the pattern features. The limit on the size of the pattern features that can be formed for a given process is partially determined by the wavelength of the radiation that is used. For a given wavelength and lithographic apparatus, it is not possible to form patterned features below a given size. However, due to the demand to form devices with patterned features as small as possible, it is usual to operate a lithographic system as close to the limit as possible. When operating a lithographic process close to the resolution limit, diffraction effects may cause spurious artifacts to appear in the pattern of radiation projected onto a substrate, e.g., spurious features which appear on the pattern of radiation exposed on the substrate, but which were not part of the pattern that was desired to be formed on the substrate.
Conventional devices have simulated the spurious effects and to modify the pattern set by the patterning device such that, once the spurious effects are taken into account, the actual pattern of radiation exposed on the substrate is as close as possible to the actual pattern desired. In addition to altering the pattern provided by the patterning device, other operational settings of the lithographic apparatus have an affect on the generation of spurious pattern features. Other conventional devices have taken such settings into account when attempting to model the spurious effects in order to predict the optimum design for the patterning device and the optimum operational settings of the lithographic apparatus in order to expose the required pattern of radiation on the substrate.
However, simulation techniques for predicting the spurious effects are not precise. Accordingly, it is typically necessary to use such a simulation technique to predict a pattern for the patterning device, expose a substrate using the predicted pattern, process the substrate, inspect the resulting pattern formed on the substrate in order to determine how it differs from the desired pattern and then use this information to improve the simulation of the spurious effects in order to provide a revised pattern for the patterning device. This process may need to be repeated several times until a satisfactory pattern for the patterning device is provided. Such a procedure is time-consuming and expensive, especially if a reticle is used as the patterning device, because manufacturing reticles is expensive and a new reticle must be manufactured for each revision of the pattern for the patterning device.
Therefore, what is needed is a system and method for ensuring that a desired pattern of radiation is exposed on a substrate without requiring time-consuming and expensive procedures.